Infectious disease link to low IQ

US researchers have come up with a hypothesis about what detracts from or improves cognitive functioning, the sort of intelligence that is measured by an IQ test.

A team of scientists at the University of New Mexico says that the prevalence of infectious diseases, particularly those involving parasites, may have a negative impact on intelligence.

Publishing their findings in the Proceedings of the Royal Society, Christopher Eppig, Corey Fincher and Randy Thornhill base their theory on assumptions about how much energy the body can produce, and how much the brain is able to burn.

The brain typically uses up around 25% of the energy produced when an adult metabolises food, while it commands 87% of the total energy budget in infants, with the percentage falling gradually as a child develops.

The researchers set out to discover whether excessive infections might correlate with impaired cognitive development worldwide.

They scanned IQ data from around the world, and found that high levels of infectious disease were indeed linked with lower average national intelligence. However, they emphasised that no causal relationship was implied.

"A developing human will have difficulty building a brain and fighting off infectious diseases at the same time, as both are very metabolically costly tasks," they write.

They said that the correlations between high infectious disease burdens and lower IQs were robust worldwide, making infectious disease the most powerful predictor of national IQ when various other factors including temperature, national economic growth and education levels were controlled for.

Calling their theory the "parasite-stress hypothesis", the researchers then used it to explain the worldwide distribution of intelligence.

"If an individual cannot meet these energetic demands while the brain is growing and developing, the brain's growth and developmental stability will suffer," they wrote.

They said that this could happen because the body had to use energy to replace tissue lost to parasites like flukes or because intestinal parasites caused the loss of nutrients through diarrhoea.

Energy was also lost when viruses used the host's cellular machinery and macromolecules to reproduce themselves, and when the immune system was activated to fight off infection.

They said diarrhoeal diseases, among the top killers of children worldwide, probably imposed the most serious cost on the energy budget of a growing child, denying the brain the energy it needed to develop cognitively.